The present invention relates to the field of personal identification and verification, and, more particularly, to the field of fingerprint sensing and processing.
Fingerprint sensing and matching is a reliable and widely used technique for personal identification or verification. In particular, a common approach to fingerprint identification involves scanning a sample fingerprint or an image thereof and storing the image and/or unique characteristics of the fingerprint image. The characteristics of a sample fingerprint may be compared to information for reference fingerprints already in a database to determine proper identification of a person, such as for verification purposes.
A typical electronic fingerprint sensor is based upon illuminating the finger surface using visible light, infrared light, or ultrasonic radiation. The reflected energy is captured with some form of camera, for example, and the resulting image is framed, digitized and stored as a static digital image. For example, U.S. Pat. No. 4,210,899 to Swonger et al. discloses an optical scanning fingerprint reader cooperating with a central processing station for a secure access application, such as admitting a person to a location or providing access to a computer terminal. U.S. Pat. No. 4,525,859 to Bowles similarly discloses a video camera for capturing a fingerprint image and uses the minutiae of the fingerprints, that is, the branches and endings of the fingerprint ridges, to determine a match with a database of reference fingerprints.
Unfortunately, optical sensing may be affected by stained fingers or an optical sensor may be deceived by presentation of a photograph or printed image of a fingerprint rather than a true live fingerprint. In addition, optical schemes may require relatively large spacings between the finger contact surface and associated imaging components. Moreover, such sensors typically require precise alignment and complex scanning of optical beams. Accordingly, optical sensors may thus be bulky and be susceptible to shock, vibration and surface contamination. Accordingly, an optical fingerprint sensor may be unreliable in service in addition to being bulky and relatively expensive due to optics and moving parts.
U.S. Pat. No. 4,353,056 to Tsikos discloses another approach to sensing a live fingerprint. In particular, the patent discloses an array of extremely small capacitors located in a plane parallel to the sensing surface of the device. When a finger touches the sensing surface and deforms the surface, a voltage distribution in a series connection of the capacitors may change. The voltages on each of the capacitors is determined by multiplexor techniques. Unfortunately, the resilient materials required for the sensor may suffer from long term reliability problems. In addition, multiplexing techniques for driving and scanning each of the individual capacitors may be relatively slow and cumbersome. Moreover, noise and stray capacitances may adversely affect the plurality of relatively small and closely spaced capacitors.
Significant advances have been made in the area of integrated circuit fingerprint sensor, as disclosed, for example, in U.S. Pat. Nos. 5,828,773 and 5,862,248, both assigned to the assignee of the present invention. The disclosed sensors are based upon generating an electric field which can sense the ridges of a fingerprint despite contamination, skin surface damage, and other factors. The sensor is relatively compact and rugged. The sensing die may be mounted on a leadframe so that the conductive pins extend outwardly from side edges of the package.
As disclosed in U.S. Pat. No. 5,862,248 to Salatino et al., an electrically conductive ring may be formed surrounding an opening in the encapsulating package. The integrated circuit die is exposed through the opening. The conductive ring may be used to drive the finger of the user to produce the fingerprint image from a plurality of sensing electrodes on the surface of the integrated circuit. Electrical contact between the conductive ring and the integrated circuit may be established by positioning the ring on an insulating layer on the die and forming one or more conductor filled vias through the insulating layer.
U.S. Pat. No. 5,940,526 to Setlak et al. discloses additional advances in the area of electric field fingerprint sensors. In particular, two electrically conductive rings are provided on the exterior upper surface of the housing. One ring may be used to discharge accumulated electrical charge from the finger of the user. The other electrode can be used to sense finger contact to thereby wake-up the device and supply power thereto. Connections from the integrated circuit die to the external rings may be made by conductor filled vias extending through the housing.
Unfortunately, the electric field fingerprint sensors as described above may still be susceptible to ESD events. This potential problem is made more difficult since the user""s finger directly contacts a relatively thin dielectric layer over the array of sensing elements. Accordingly, to facilitate more widespread use and reliability, there are still additional enhancements needed in the area of coping with ESD.
In view of the foregoing background, it is therefore an object of the present invention to provide a fingerprint sensor package which is resistant to transients, such as ESD events.
This and other objects, features and advantages in accordance with the present invention are provided by a fingerprint sensor package comprising a fingerprint sensing integrated circuit within a housing and comprising a plurality of electric field sensing electrodes and processing circuitry connected thereto; at least one external electrode carried by the housing for finger contact; an electrostatic discharge (ESD) circuit within the housing and connected to the at least one external electrode; and an isolation circuit within the housing. The isolation circuit is connected to the at least one line of the processing circuitry for external connection. The external electrode and ESD circuit, as well as the line isolation circuit protect the integrated circuit against damage by voltage transients, such as ESD.
The at least one line for external connection may comprise at least one power supply line. Accordingly, the isolation circuit may comprise a power supply isolation circuit connected to the at least one power supply line. The power supply isolation circuit may include a DC power modulator, and a DC power demodulator magnetically coupled thereto. In an alternate embodiment, a respective resistor may be coupled in series with one or both of the power supply lines instead of the DC power modulator and demodulator.
The at least one line for external connection may alternately or additionally comprise at least one data signal line. Accordingly, the isolation circuit may comprise a data signal isolation circuit connected to the at least one data signal line, such as including one or more opto-isolators. In another embodiment, the data signal isolation circuit may include a respective resistor connected in series with one or more of the data signal lines instead of the opto-isolators.
In accordance with another aspect of the invention, the at least one external electrode may include first and second external electrodes. The first electrode may be connected to the electrostatic discharge circuit. In addition, the processing circuit may further comprise a wake-up circuit for selectively powering predetermined circuit portions responsive to sensing finger contact with the second external electrode. Moreover, the housing may define a recess therein to guide a finger of a user along a predetermined path of travel to contact the first external electrode before the second external electrode. Accordingly, power is not applied until the electrostatic energy has been discharged through the first external electrode.
The fingerprint sensing integrated circuit is preferably mounted on a circuit substrate. The electric field sensing electrodes define a finger sensing area which is directly contacted by a finger of the user. The fingerprint sensor package may also include at least one power supply rail on the circuit substrate laterally outside of the finger sensing area. Accordingly, another potentially damaging discharge path for ESD is accounted for in the invention.
A method aspect is for making a fingerprint sensor package comprising the step of forming a fingerprint sensing integrated circuit including a plurality of electric field sensing electrodes and processing circuitry connected thereto, with the processing circuit comprising at least one line for external connection. The method also includes positioning the fingerprint sensing integrated circuit within a housing, and forming at least one external electrode on the housing for contact by a finger of a user. The method also preferably includes providing an electrostatic discharge circuit within the housing and connected to the at least one external electrode for protecting the fingerprint sensing integrated circuit; and isolating the at least one line for external connection for further protecting the fingerprint sensing integrated circuit.
The at least one line for external connection may comprise at least one power supply line, and the step of isolating comprises connecting a power supply isolation circuit to the at least one power supply line. The at least one line for external connection may alternately or additionally comprise at least one data signal line. Thus, the step of isolating may comprise connecting a data signal isolation circuit to the at least one data signal line.
The step of forming the at least one external electrode preferably comprises forming first and second external electrodes. The first electrode is preferably connected to the electrostatic discharge circuit. The method may also include the step of selectively powering portions of the processing circuitry responsive to sensing finger contact with the second external electrode. In addition, forming the fingerprint sensing integrated circuit may further include forming at least one power rail on a semiconductor substrate laterally adjacent a sensing area defined by the electric field sensing electrodes.